EE 232 Lightwave DevicesgLecture 20: Avalanche Photodiode (APD)Rdi Ch 144Reading: Chuang 14.4Instructor: Ming C. WuUniversity of California, BerkeleyElectrical Engineering and Computer Sciences DeptElectrical Engineering and Computer Sciences Dept.EE232 Lecture 20-1©2008. University of CaliforniaAvalanche Photodiode (APD)EE232 Lecture 20-2©2008. University of CaliforniaTypical APD Structure:Separate Absorption and Multiplication (SAM) APDEE232 Lecture 20-3©2008. University of CaliforniaIdeal APD: Injection Impact Ionization Only()()ndJ xJ1()(): electron ionization coefficient [cm ]nnnnJxdxαα−=Electron current distribution along the field:() (0)nxJx J eα=() (0)Multiplication factor:nnJx J e()(0)nWnnnJWMeJα==EE232 Lecture 20-4©2008. University of CaliforniaPractical APD: Both Electron and Hole produce Impact Ionizations()() ()ndJ xJx Jxαβ⎧=+⎪⎪() ()()() ()nn pppnn ppJx JxdxdJ xJx Jxdxαβαβ+⎪⎪⎨⎪−= +⎪⎩()nJx11: electron ionization coefficient [cm ]: hole ionization coefficient [cm ]npdxαβ−−⎩()() () 0( ) ( ) constantnpdJx JxdxJx Jx J+=+==()( ) ( ) constant()()npnnpn pJx Jx JdJ xJx Jdxαβ β+==−− =EE232 Lecture 20-5©2008. University of CaliforniaMultiplication Factor()Multiplication factor:1(0)nWJMJ==1104×0.2k=()'0(0)1'1npWxnnJdx eMαβα−−−∫1103×Mn 0 x, ()Mn02 ()nM0k=0.9k=()()11npnWnnpMeαβααββ−−=−−−10100Mn0.2x, ()Mn 0.9 x, ()0k=()npnpWnpeαβαβαββ−−−=−0 1 2 3 4 51xWαDefine 1pnkkMβα=−nWαEE232 Lecture 20-6©2008. University of California(1 )nnkWMekα−−=−Response TimeRtiGi B d idthP d tResponse time = transit time in absorption region + multiplication timeGain-Bandwidth Product:11evGBW M MptmnnmMkW MkWWττττ=+⎧=+≈⎪22constant2ennmneGBW M MMkWvGBWkWπτ π×=⋅ =⋅×==meh eabsthvvvWvτ⎪⎪⎨⎪=⎪⎩2kWπUsually hmt mvττ ττ⎪⎩→≈EE232 Lecture 20-7©2008. University of CaliforniaNoise FigureExcess noise factor F2(due to fluctuation in gain) :1(1 ) 2MFkMk⎛⎞==+−−⎜⎟⎜⎟2(1 ) 2Noise Figure:10lnnFkMkMMNF F+⎜⎟⎜⎟⎝⎠10logNF F=Small has small under high gain Minimum noise figure = 3 dB occurs when 0nkF Mk =EE232 Lecture 20-8©2008. University of CaliforniaAPD Noises22Signal is amplified by the average gain :1Mq⎛⎞1222212Shot noise is amplified by :p optqiP MMηω⎛⎞=⎜⎟⎝⎠=1109×11010×11011×11012×Slope=1at high ipNR()()22222SpBDpBDieIIIMdveIIIFMdv=++=++1106×1107×1108×110×Slope=2at lowiSN()24pBDBTedvkT viRΔ=1106−× 1105−× 1104−× 1103−× 0.01 0.1 11104×1105×ipat low ipPhotocurrenti()2221242optBqPMSNRkT vIIIFMdηω⎛⎞⎜⎟⎝⎠=Δ=Photocurrent, ipEE232 Lecture 20-9©2008. University of California()2BpBDeIIIFMdvR+++SNR Comparison of p-i-n and APD22212APD: 4optqPMSNRkT vηω⎛⎞⎜⎟⎝⎠=Δ=()242p-i-n: set 1, 1BpBDkT veI I I FM dvRMFΔ+++==()2124optqPSNRkT vηω⎛⎞⎜⎟⎝⎠=Δ=ip81109×11010×()42BpBDkT veI I I dvRΔ++ +1105×1106×1107×1108×Slope=1at highiR101001103×1104×Slope=2at low ipat high ipSNREE232 Lecture 20-10©2008. University of California1109−× 1107−× 1105−× 1103−× 0.11ipPhotocurrent,